The present invention relates generally to magnetically operated switches for opening and closing circuits. More particularly, the invention relates to improved high voltage reed switches that reduce the effect of electrostatic pull between the open reed contacts thereof.
A conventional reed switch includes two flexible ferromagnetic reed switch contacts in a parallel partially overlapping arrangement, one contact sealed in each end of an hermetically sealed glass tube. When such a sealed reed contact arrangement is subjected to a magnetic field a flux path is established axially along one reed contact, across an air gap between the two reed contacts and axially along the second. The overlapping ends of the two reed contacts are thus oppositely polarized and attracted to one another, operating the reed switch. In practice, any given reed switch will be designed to operate and release at preselected magnetic flux densities by proper selection of reed contact materials and size of the gap between the overlapping portions of the reed contacts.
High voltage reed switches, for example, those handling more than 500 volts, are commonly known and used for various functions, such as in the operation of defibrillators. However, as high voltage is applied to an open reed switch, an electrostatic voltage is often produced across the open contacts. This electrostatic voltage creates an electrostatic attraction which pulls the contacts closer together thereby reducing the voltage hold-off of the switch or in some cases closing the contacts causing them to short out.
When the contacts close, the electrostatic voltage is temporarily eliminated and the contacts immediately reopen. This undesired closing and opening may be continuously repeated while voltage is applied to the open switch and can cause unnecessary wear on the reed contacts. Hence, conventional high voltage reed switches do not have low ampere turn sensitivity because at moderately high voltage the contacts are pulled so close together that they arc over or actually close.
Until now, no reed switch has been provided with an effective means for reducing the effect of the electrostatic attraction between the open reed contacts thereof while voltage is applied to the switch. For the foregoing reasons, there is a need for improved high voltage reed switches that are simple, economical and effective.